network protocols handbook.pdf

91Protocols GuideTCP/IP - Network Layer ProtocolsProtocol NamePIM-SM: Protocol IndependentMulticast-Sparse ModeProtocol DescriptionProtocol Independent Multicast (PIM) has two modes: SparseMode and Dense Mode. We focus on the Sparse Mode in thisdocument.PIM-SM is a protocol for efficiently routing to multicast groupsthat may span wide-area (WAN and inter-domain) internets,while PIM-DM is mainly for LAN. The protocol is not dependenton any particular unicast routing protocol, and is designed tosupport sparse groups. It uses the traditional IP multicast modelof receiver-initiated membership, supports both shared andshortest-path trees, and uses soft-state mechanisms to adaptto changing network conditions. It can use the route informationthat any routing protocol enters into the multicast Routing InformationBase (RIB). Examples of these routing protocols includeunicast protocols such as the Routing Information Protocol (RIP)and Open Shortest Path First (OSPF), but multicast protocolsthat populate the routing tables—such as the Distance VectorMulticast Routing Protocol (DVMRP)—can also be used.PIM-SM was designed to support the following goals:• Maintain the traditional IP multicast service model ofreceiver-initiated multicast group membership. In thismodel, sources simply put packets on the first-hop Ethernet,without any signaling. Receivers signal to routersin order to join the multicast group that will receive thedata.• Leave the host model unchanged. PIM-SM is a routerto-routerprotocol, which means that hosts don’t have tobe upgraded, but that PIM-SM-enabled routers must bedeployed in the network.• Support both shared and source distribution trees. Forshared trees, PIM-SM uses a central router, called theRendezvous Point (RP), as the root of the shared tree.All source hosts send their multicast traffic to the RP,which in turn forwards the packets through a commontree to all the members of the group. Source trees directlyconnect sources to receivers. There is a separatetree for every source. Source trees are consideredshortest-path trees from the perspective of the unicastrouting tables. PIM-SM can use either type of tree orboth simultaneously.• Maintain independence from any specific unicast routingprotocol (see above).• Use soft-state mechanisms to adapt to changing networkconditions and multicast group dynamics. Softstatemeans that, unless it is refreshed, the router’sstate configuration is short-term and expires after a certainamount of time.Currently, there are two versions of PIM-SM. We focus on version2, which is widely deployed.Protocol StructureThe protocol format of PIM-DM is the same as that of PIM-SM:PIM versionTypeReserved(Address length)Checksum• PIM version – The current PIM version is 2.• Type -- Types for specific PIM messages.• Address length -- Address length in bytes. The lengthof the address field throughout, in the specific message.• Reserved - The value of this field is set to 0, ignoreon receipt• Checksum - The 16-bit field is the one’s complementsum of the entire PIM message.Related protocolsPIM-DM, ICMP, RIP, OSPF, DVMRP, IS-IS, BGP, IGRP, EIGRPSponsor SourcePIM-SM is defined by IETF (http://www.ietf.org) RFC2362.Referencehttp://www.javvin.com/protocol/rfc2362.pdfPIM-SM: Protocol Specification